Various types of fire detection systems are known in the prior art. Such systems often include a plurality of detectors spaced-apart from one another and located in a region for which protection is to be provided. The detectors are coupled, via transmission lines of one type or another, to multiplexing circuitry and/or then to central fire control panels. The fire control panels in large buildings are usually monitored 24 hours a day and provide visual and/or audible indications of the existence of an alarm condition as well as the general location in the region being monitored.
Many pre-existing fire alarm systems have been wired using non-twisted pair or non-shielded wire. Earlier known systems, particularly those which are analog systems, have provided very reasonable levels of noise immunity without using twisted pair or shielded wire. Newer systems very often use higher frequency digital transmission protocols which are more susceptible to noise. One such system is disclosed in Tice et al. U.S. Pat. No. 4,916,432 assigned to Pittway Corporation.
In the prior art, devices have been used which can distinguish between valid electrical signals and unwanted noise by sensing the durations of the respective signals. However, noise impulses which are approximately the same duration as a valid electrical signal often, with prior art products, cannot be distinguished from the valid signal. Hence, the noise which cannot be distinguished, can result in altering an otherwise valid transmission or perhaps creating a false alarm condition.
One approach to minimizing noise problems of transmission lines is to use twisted pair or shielded transmission cables. However, twisted pair or shielded wire is more expensive than non-twisted or non-shielded wire.
In addition to the higher cost of higher performance transmission line cabling, it would be very desirable to be able to use the existing non-twisted pair, non-shielded wire in upgrading existing systems to higher performance levels. Thus, substantial commercial advantages can be obtained by being able to use existing non-twisted pair, non-shielded wire in combination with state-of-the-art high speed digital transmission systems while at the same time providing enhanced noise immunity.